0,0 → 1,225 |
/* rwsem.h: R/W semaphores implemented using XADD/CMPXCHG for i486+ |
* |
* Written by David Howells (dhowells@redhat.com). |
* |
* Derived from asm-x86/semaphore.h |
* |
* |
* The MSW of the count is the negated number of active writers and waiting |
* lockers, and the LSW is the total number of active locks |
* |
* The lock count is initialized to 0 (no active and no waiting lockers). |
* |
* When a writer subtracts WRITE_BIAS, it'll get 0xffff0001 for the case of an |
* uncontended lock. This can be determined because XADD returns the old value. |
* Readers increment by 1 and see a positive value when uncontended, negative |
* if there are writers (and maybe) readers waiting (in which case it goes to |
* sleep). |
* |
* The value of WAITING_BIAS supports up to 32766 waiting processes. This can |
* be extended to 65534 by manually checking the whole MSW rather than relying |
* on the S flag. |
* |
* The value of ACTIVE_BIAS supports up to 65535 active processes. |
* |
* This should be totally fair - if anything is waiting, a process that wants a |
* lock will go to the back of the queue. When the currently active lock is |
* released, if there's a writer at the front of the queue, then that and only |
* that will be woken up; if there's a bunch of consequtive readers at the |
* front, then they'll all be woken up, but no other readers will be. |
*/ |
|
#ifndef _ASM_X86_RWSEM_H |
#define _ASM_X86_RWSEM_H |
|
#ifndef _LINUX_RWSEM_H |
#error "please don't include asm/rwsem.h directly, use linux/rwsem.h instead" |
#endif |
|
#ifdef __KERNEL__ |
#include <asm/asm.h> |
|
/* |
* The bias values and the counter type limits the number of |
* potential readers/writers to 32767 for 32 bits and 2147483647 |
* for 64 bits. |
*/ |
|
#ifdef CONFIG_X86_64 |
# define RWSEM_ACTIVE_MASK 0xffffffffL |
#else |
# define RWSEM_ACTIVE_MASK 0x0000ffffL |
#endif |
|
#define RWSEM_UNLOCKED_VALUE 0x00000000L |
#define RWSEM_ACTIVE_BIAS 0x00000001L |
#define RWSEM_WAITING_BIAS (-RWSEM_ACTIVE_MASK-1) |
#define RWSEM_ACTIVE_READ_BIAS RWSEM_ACTIVE_BIAS |
#define RWSEM_ACTIVE_WRITE_BIAS (RWSEM_WAITING_BIAS + RWSEM_ACTIVE_BIAS) |
|
/* |
* lock for reading |
*/ |
static inline void __down_read(struct rw_semaphore *sem) |
{ |
asm volatile("# beginning down_read\n\t" |
LOCK_PREFIX _ASM_INC "(%1)\n\t" |
/* adds 0x00000001 */ |
" jns 1f\n" |
" call call_rwsem_down_read_failed\n" |
"1:\n\t" |
"# ending down_read\n\t" |
: "+m" (sem->count) |
: "a" (sem) |
: "memory", "cc"); |
} |
|
/* |
* trylock for reading -- returns 1 if successful, 0 if contention |
*/ |
static inline int __down_read_trylock(struct rw_semaphore *sem) |
{ |
long result, tmp; |
asm volatile("# beginning __down_read_trylock\n\t" |
" mov %0,%1\n\t" |
"1:\n\t" |
" mov %1,%2\n\t" |
" add %3,%2\n\t" |
" jle 2f\n\t" |
LOCK_PREFIX " cmpxchg %2,%0\n\t" |
" jnz 1b\n\t" |
"2:\n\t" |
"# ending __down_read_trylock\n\t" |
: "+m" (sem->count), "=&a" (result), "=&r" (tmp) |
: "i" (RWSEM_ACTIVE_READ_BIAS) |
: "memory", "cc"); |
return result >= 0 ? 1 : 0; |
} |
|
/* |
* lock for writing |
*/ |
static inline void __down_write_nested(struct rw_semaphore *sem, int subclass) |
{ |
long tmp; |
asm volatile("# beginning down_write\n\t" |
LOCK_PREFIX " xadd %1,(%2)\n\t" |
/* adds 0xffff0001, returns the old value */ |
" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" |
/* was the active mask 0 before? */ |
" jz 1f\n" |
" call call_rwsem_down_write_failed\n" |
"1:\n" |
"# ending down_write" |
: "+m" (sem->count), "=d" (tmp) |
: "a" (sem), "1" (RWSEM_ACTIVE_WRITE_BIAS) |
: "memory", "cc"); |
} |
|
static inline void __down_write(struct rw_semaphore *sem) |
{ |
__down_write_nested(sem, 0); |
} |
|
/* |
* trylock for writing -- returns 1 if successful, 0 if contention |
*/ |
static inline int __down_write_trylock(struct rw_semaphore *sem) |
{ |
long result, tmp; |
asm volatile("# beginning __down_write_trylock\n\t" |
" mov %0,%1\n\t" |
"1:\n\t" |
" test " __ASM_SEL(%w1,%k1) "," __ASM_SEL(%w1,%k1) "\n\t" |
/* was the active mask 0 before? */ |
" jnz 2f\n\t" |
" mov %1,%2\n\t" |
" add %3,%2\n\t" |
LOCK_PREFIX " cmpxchg %2,%0\n\t" |
" jnz 1b\n\t" |
"2:\n\t" |
" sete %b1\n\t" |
" movzbl %b1, %k1\n\t" |
"# ending __down_write_trylock\n\t" |
: "+m" (sem->count), "=&a" (result), "=&r" (tmp) |
: "er" (RWSEM_ACTIVE_WRITE_BIAS) |
: "memory", "cc"); |
return result; |
} |
|
/* |
* unlock after reading |
*/ |
static inline void __up_read(struct rw_semaphore *sem) |
{ |
long tmp; |
asm volatile("# beginning __up_read\n\t" |
LOCK_PREFIX " xadd %1,(%2)\n\t" |
/* subtracts 1, returns the old value */ |
" jns 1f\n\t" |
" call call_rwsem_wake\n" /* expects old value in %edx */ |
"1:\n" |
"# ending __up_read\n" |
: "+m" (sem->count), "=d" (tmp) |
: "a" (sem), "1" (-RWSEM_ACTIVE_READ_BIAS) |
: "memory", "cc"); |
} |
|
/* |
* unlock after writing |
*/ |
static inline void __up_write(struct rw_semaphore *sem) |
{ |
long tmp; |
asm volatile("# beginning __up_write\n\t" |
LOCK_PREFIX " xadd %1,(%2)\n\t" |
/* subtracts 0xffff0001, returns the old value */ |
" jns 1f\n\t" |
" call call_rwsem_wake\n" /* expects old value in %edx */ |
"1:\n\t" |
"# ending __up_write\n" |
: "+m" (sem->count), "=d" (tmp) |
: "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS) |
: "memory", "cc"); |
} |
|
/* |
* downgrade write lock to read lock |
*/ |
static inline void __downgrade_write(struct rw_semaphore *sem) |
{ |
asm volatile("# beginning __downgrade_write\n\t" |
LOCK_PREFIX _ASM_ADD "%2,(%1)\n\t" |
/* |
* transitions 0xZZZZ0001 -> 0xYYYY0001 (i386) |
* 0xZZZZZZZZ00000001 -> 0xYYYYYYYY00000001 (x86_64) |
*/ |
" jns 1f\n\t" |
" call call_rwsem_downgrade_wake\n" |
"1:\n\t" |
"# ending __downgrade_write\n" |
: "+m" (sem->count) |
: "a" (sem), "er" (-RWSEM_WAITING_BIAS) |
: "memory", "cc"); |
} |
|
/* |
* implement atomic add functionality |
*/ |
static inline void rwsem_atomic_add(long delta, struct rw_semaphore *sem) |
{ |
asm volatile(LOCK_PREFIX _ASM_ADD "%1,%0" |
: "+m" (sem->count) |
: "er" (delta)); |
} |
|
/* |
* implement exchange and add functionality |
*/ |
static inline long rwsem_atomic_update(long delta, struct rw_semaphore *sem) |
{ |
return delta + xadd(&sem->count, delta); |
} |
|
#endif /* __KERNEL__ */ |
#endif /* _ASM_X86_RWSEM_H */ |